Quantifying aquatic plant commonness and co‐occurrence across scales to support ecological understanding and management

Abstract Understanding how species differ in commonness and coexistence with other species is foundational to ecology and ecosystem management. Species can be regionally and locally common or rare at either scale. Similarly, species can be associated with diverse communities and locally co‐occur with numerous neighbours or occupy species‐poor habitats. These relationships are important for understanding aquatic plant communities, which are sensitive to environmental degradation and invasion. We analysed aquatic plant surveys from 1658 lakes across Minnesota and Wisconsin, USA, collected over two decades (2000–2022) and encompassing nearly 1 million sampling points. For 106 focal taxa, we quantified commonness as occupancy (at regional and local scales) and co‐occurrence as diversity fields (the mean co‐occurring species richness of lakes or sampling locations where each focal species occurred). We used statistical models that incorporated environmental, spatial and temporal covariates to correct for biased sampling and isolate community processes from other influential factors, and leveraged the temporal span of the data to investigate interannual variability in commonness and co‐occurrence. All combinations of high × low regional and local occupancy were represented among focal taxa, as were all combinations of regional and local diversity fields, but certain patterns were more prevalent. Species in high‐diversity lakes were typically regionally and locally uncommon; species in low‐diversity lakes tended to be common at both scales. Invasive species were common and associated with species‐poor lakes; highly conservative species exhibited the opposite patterns. Interannual temporal variability was higher for occupancy than diversity fields and decreased with increasing occupancy. Synthesis . Macrophyte commonness and co‐occurrence were quantified to support objective classifications of species distributions in Midwestern lakes. This framework provides a practical tool for conservation prioritization (e.g. identifying species that are rare at regional and local scales that may be more susceptible to local disturbance), invasive species risk assessment (e.g. identifying high‐risk species common at multiple scales or measuring change over time to infer impact) and restoration planning (e.g. identifying ‘workhorse’ species with high regional occupancy that are not restricted to species‐rich lakes, implying broader environmental tolerance). This approach is transferable to other systems with sufficiently rich biodiversity monitoring data.